Your Gradient Factors?

[Pressurehead]

Worth a read:
http://www.swiss-cave-diving.ch/PDF-dateien/Oxygen-Hamilton.pdf
Some of the Deco dives we did on mixed gases years ago, it is amazing [or good luck, or good planning ?] We survived and not bent. [Young dumb and super fit].
Some decompression schedules [I am sure] were an educated guess by our diving supervisors. You did NOT argue with them.

 

[elmo]

The concept of average depth for bottom time portion of the dive, and using that depth to determine the deco schedule (or NDL for that matter) using tables is not at all radical. If you think it is crazy snake oil, could you please humour me and test using your preferred deco planner tool and settings (my choice is Subsurface with Bühlmann gf 60/85 but that's just me)
A) Square profile of any depth, bottom time (e.g. 4min to descend to 50m, leave bottom at 30min runtime, using 18/45 bottom gas and 50% deco gas, Subsurface tells me I get out of the water at 76min)
B) A deep-to-shallow profile of same average bottom depth and time (e.g. 4min to descend to 60m, stay for 10min, ascend to 40m over 6min, stay for 10min, leave bottom at 30min runtime, same gassed as above. I get out of the water at 72min - 4min less deco than square profile A)
C) A shallow-to-deep profile "reverse of above" (e.g. 4min to descend to 40m, stay for 10min, descend to 60m over 6min, stay for 10min, leave bottom at 30min. I get out of the water at 80min - 4min more deco than square profile A)
D) A sawtooth profile of same average depth and bottom time (e.g. drop to 60m over 4min, ascend to 40m over 7min, back to 60m over 7min, back to 40m over 7m, descend to 50m over 5min, leave bottom at 30min. I get out of the water at 75min - just 1min less than square profile A).

Let me guess. Whatever you chose, case B (deep to shallow) required a bit less deco than case A (square), case C (shallow to deep) required a bit more deco, and case D (sawtooth) required about the same deco as case A.

You can use maths to prove that assuming exponential decay of gas absorption for tissues with a certain half-time that deep to shallow will always end up with less inert gas absorption than a square profile, and shallow to deep will absorb more.

So what? The upshot is that for a deep to shallow profile (most common for multi-level dives) you can safely assume it is similar to a square profile of same average depth and bottom time. You'll take a little bit more time doing deco than an optimized plan, but it will work. You should avoid (or otherwise compensate) shallow to deep profiles when using depth averaging. There are other issues with sawtooth profiles not covered by Bühlmann or VPM models, but they are inevitable in many caves and some wrecks, and consensus seems to be that they are ok if the sawtoothing is not too rapid.

There are pseudo-averaging techniques that attempt to compensate in the deep-to-shallow vs shallow-to-deep bias.

Of course, planning based on an accurate profile will give a more accurate deco schedule, and should be done where feasible. Computer can/should be relied upon too. But averaging can be useful too either as a backup, or in anticipating changes to what deco will be required during a dive.

 

[nadwidny]

Nothing wrong with Hamilton's tables, they were impressive.
But they are not the point.
The point is that most of the recreational diving world was NOT doing trimix dives using Hamilton's tables.
Most of diving world (pre-computers) was using tables, like Navy or others that use depth, not average depth.

Um well, the people I dove with were using average depth. And I wasn't in Puget Sound.

You need to broaden your scope more. Your view is pretty limited.

 

[Ayisha]

The GUE tables I've seen do not use average depth. Has something changed?

Yes, a little. My GUE Nitrox and TriOx Minimum Deco Table and On-the-Fly Minimum Deco table from 2014 both involved calculating 50% of the *Max* Depth to determine the first MD stop and ascent profile. Not sure how old those tables are. When you're planning a dive, that's pretty much the average depth.

The Standard Operating Procedures vol. 3.0, from 2017, which is current, states "Calculate 50% of the average depth of the dive".

Note that those 2 tables are for MD dives up to 100 feet.
Tech 1 and 2 use specified GF's and a different percentage of average depth to calculate the first stop.

 

[beester]

Well since I apparently started the discussion on average depth. How I use it is quite simple:

- Average depth for diveplanning: when planning dives with a square profile (typical wreck dives) I do use average depth and not max depth to plan the dive. The variance is not as great as a multilevel dive so this is perfectly fine to do. Let's say you have a wreck with a bottom at 60m (200ft) and a top at 48m (157ft). I can plan this dive stating I'll dive it with an avg depth of 51m and during the dive I can adjust based on the actual dive (we stay longer at the deeper part)... since the variance in avg depth is not very big (3-5m for a dive like this resulting in 5-10 min more or less deco) this is perfectly safe to do. Of course if you have a divecomputer or gauge which can reset the average depth once you arrive at depth, that's even better obviously. (taking the fallible human brain out of the equation).

- Average depth as a double check of the conservatism of your deco: As I already stated there is a direct link between your hi GF % and average depth. So it's just an easy way to double check where you are at during your dive. I always have my AVG depth on my computer + GF %, but actually having Avg depth is enough. For my typical dives I won't ascend from 9-6m for O² switch before I reach a certain avg depth, and I also double check my avg depth before I start the ascend to the surface after deco is finished.

I'm sorry that I don't go into more detail, the reason is that it's just a double check... better to have your GF% on your computer main screen and use that PLUS this works well for deeper square profile dives, but will get you in trouble for shallower dives (specially if they are longer, for example 30m-1h dive).

Cheers

B

 

[jale]

- Average depth as a double check of the conservatism of your deco: As I already stated there is a direct link between your hi GF % and average depth. So it's just an easy way to double check where you are at during your dive. I always have my AVG depth on my computer + GF %, but actually having Avg depth is enough. For my typical dives I won't ascend from 9-6m for O² switch before I reach a certain avg depth, and I also double check my avg depth before I start the ascend to the surface after deco is finished.
B

Yes. As Raph and I said it before, it was a "trick" to gauge your position and as Beester says, it is personal.
I have been using this for a few years and I wonder who is using it...maybe another thread should be opened

 

[JonG1]

@beester I'm intrigued as to why longer "shallower" dived may give rise to a higher incidence of dcs than deeper shorter dives.

Is it because more dives are conducted in the normoxic range, a failure in the models, or that the durations don't extrapolate correctly etc?

 

[beester]

@beester I'm intrigued as to why longer "shallower" dived may give rise to a higher incidence of dcs than deeper shorter dives.

Is it because more dives are conducted in the normoxic range, a failure in the models, or that the durations don't extrapolate correctly etc?

Heya JonG, I can't give a very good answer to that... just some theories from a non scientist (I'm a history major, and have not the brains or the background to make meaningful comments on deco models or the biochemistry going on in a human body during decompression).

But some thoughts:

- Experience: I've seen related to number of dives much more deco accidents/incidents in shallow deco dives (not talking about recreational dives) vs deeper dives on trimix with a shorter or even equal bottom time. A typical dive would be 50-60 minutes at 30m (100ft). Not only real accidents (with people need to have hyperbaric treatment for perfectly normal ascend profiles) but also divers feeling more tired, lackluster, etc (asymptomatic dcs). I've been witness to 4 accidents in the past 8 years diving these kind of profiles, while I've only been witness to 1 during deep(er dives) and that 1 was really related to the diver mismanaging his ascend.
- I believe this higher incidence has to have a link with the ongassing of slower tissues. In the past this could have maybe be linked to wrongly planning dives (using ratio deco which is faulty the farther you get away from a certain setpoint) but the last 7-8 years these dives have been planned only using deco software or computer, but still using the same (high) GF as for normoxic trimix dives. There has been plenty of discussion on the Low GF (aka deepstops), but that's not what I'm talking about.

I know all very aneckdotal, but like I say I'm no scientist, just a diver. I also don't defend this when discussing with other divers, but I will always err on the side of caution when planning shallow deco dives (setting the hi GF to as low a setting as 65-70).

Just my 2 cents

B

 

[rjack321]

- I believe this higher incidence has to have a link with the ongassing of slower tissues. In the past this could have maybe be linked to wrongly planning dives (using ratio deco which is faulty the farther you get away from a certain setpoint) but the last 7-8 years these dives have been planned only using deco software or computer, but still using the same (high) GF as for normoxic trimix dives. There has been plenty of discussion on the Low GF (aka deepstops), but that's not what I'm talking about.

VPM is also widely divergent from buhlmann on these depth/duration dives. Are you sure what software and settings they were using? 8 years ago I was using VPM +3 (for instance) and bent myself on ~4 hour 32% dives in the 65-105ft range (Ginnie Springs OC dives)

 

[tursiops]

You need to broaden your scope more. Your view is pretty limited.

You mean I need to include DIR/GUE/UTD as part of the broader world of recreational diving? Naw. They are outliers, a tiny fraction of the global diving activity.

My view of tables and actual depth comes from pre-computer days diving in Chicago, New England, Europe, using many kinds of tables. ALL of which clearly specify actual depth, not some sort of average depth.

One example above talked about going between 220 ft and 240 ft and calling it 230 ft. OK, that is what is called a tangent linear approximation, perfectly plausible for gas up take. But that is not the average depth of the dive, just of a little segment of it. That appears to be what the Ratio Deco folks have reinvented, the tangent-linear approximation. No magic, but definitely not the average depth of a dive, just of some segment of the dive.

The examples above that are trying to justify their position are really way off in the corner of recreational diving: trimix, 240 ft, 4h dives, O2 switch and deco, etc. Does the Navy dive manual or the NOAA diving manual or DCIEM or BSAC or anybody in the mainstream use average depth with a table?